L78M datasheet(24/47 Pages) STMICROELECTRONICS

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Applications information

L78M

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DocID2146 Rev 21

Applications information

7.1

Design considerations

The L78M series of fixed voltage regulators are designed with thermal overload protection

that shuts down the circuit when subjected to an excessive power overload condition,

internal short-circuit protection that limits the maximum current the circuit will pass, and

output transistor safe-area compensation that reduces the output short-circuit as the

voltage across the pass transistor is increased. In many low current applications,

compensation capacitors are not required. However, it is recommended that the regulator

input be bypassed with a capacitor if the regulator is connected to the power supply filter

with long wire lengths, or if the output load capacitance is large. An input bypass capacitor

should be selected to provide good high-frequency characteristics to insure stable

operation under all load conditions. A 0.33 µF or larger tantalum, mylar, or other capacitor

having low internal impedance at high frequencies should be chosen. The bypass capacitor

should be mounted with the shortest possible leads directly across the regulators input

terminals. Normally good construction techniques should be used to minimize ground loops

and lead resistance drops since the regulator has no external sense lead.

Figure 17: Fixed output regulator

Although no output capacitor is need for stability, Co improve transient response if

present. CI is required if regulator is located an appreciable distance from power

supply filter.

Figure 18: Constant current regulator

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